The present invention relates to a gas reformer for recovering hydrogen gas generated by thermal decomposition of hydrocarbon gas.
Hydrogen has been used in broad industrial fields, as basic raw material in a chemical industry, a fuel for a fuel cell or atmospheric gas for heat treatment. A representative process to cope with a small demand is reformation of hydrocarbon with steam. Since a product obtained by the reforming process contains CO, CO2 and residual H2O other than H2, it cannot be used as such for a fuel cell due to the inclusions; otherwise performance of the fuel cell would be worsened. In this regard, removal of subspecies such as CO, CO2 and residual H2O from H2 is necessitated, before the reformed product is supplied to a fuel cell
A conventional method of removing subspecies uses a hydrogen-permeating membrane made of a catalytic element such as Pdxe2x80x94Ag or Ta, which enables selective permeation of hydrogen The hydrogen-permeating membrane has been formed so far as a thin layer on a heal-resistant porous body, as disclosed in JP 63-294925 A1 and JP 1-164419 A1. Recently, feasibility of a metal body, which is perforated with holes for passage of hydrogen has been studied instead of a conventional heat-resistant porous body.
In a conventional method using a hydrogen-permeating membrane, a double-pipe 2 is located in a jacket 1, a plurality of hydrogen-separating pipes 3 each composed of a perforated body 3a and a hydrogen-permeating membrane 3b are inserted between inner and outer walls of the double-pipe 2, and a cavity of the double-pipe 2 is filled with a catalyst 4. A box-shape hydrogen-separator, which has an external surface coated with a hydrogen-permeating membrane 3b, may be used instead of the hydrogen-separating pipe 3. The catalyst may be Ni or the like supported by alumina or the like.
A fuel F is fed together with air A through a burner 5 and a burner tile 6 into an inner space of the double-pipe 2, and burnt therein. Hydrocarbon gas G to be reformed is blown together with steam through a nozzle 7 into a cavity between inner and outer walls of the double-pipe 2, and decomposed to H2 and CO2 according to a reforming reaction of CH4+2H2O=4H2+CO2 for instance.
A reaction product H2 selectively permeates through the membrane 3b into the hydrogen-separator 3, and flows out through a takeout pipe 8. Selective permeation of hydrogen H2 from a reacting zone through the hydrogen-permeating membrane 3b accelerates the reforming reaction of CH4+2H2O=4H2+CO2. A by-product CO2 is discharged as waste gas W together with excessive H2O and combustion gas through an exhaust pipe 9 to the outside.
The reforming reaction of CH4+2H2O=4H2+CO2 is accelerated at a temperature above 690xc2x0 C., and the reaction rate quickens as increase of the temperature. Another reaction of CO+H2O=CO2+H2 is exothermic on the contrary, and the reaction does not advance over 707xc2x0 C. In order to efficiently promote these reactions, the double-pipe 2 is conventionally heated with combustion heat of a fuel F in the manner such that an inner space of the double-pipe 2 is held at a temperature in a range of about 600-900xc2x0 C. with a proper temperature gradient.
Heat-resistant stainless steel is representative material for high-temperature use, but an atmosphere in the gas reformer contains steam for reformation of hydrocarbon. Such the wet atmosphere causes oxidation and intergranular corrosion of a perforated body made of a conventional heat-resistant stainless steel such as SUS410L, SUS430 or SUS304. As a result, the hydrogen-permeating membrane 3b is peeled off or cracked, and H2 gas flowing through the takeout pipe 8 reduces its purity due to inclusion of C2H2n+2, H2O and CO2.
Due to selective separation of H2 from the reacting zone through the hydrogen-permeating membrane 3b, equilibrium in the reaction of CH4+2H2O=4H2+CO2 collapses, and the reaction progresses to the rightward. Consequently a temperature necessary for the reforming reaction can be lowered to 450-600xc2x0 C. However, the reacting atmosphere is still at a high temperature. When the reformer is operated at such a high-temperature atmosphere over a long term, the hydrogen-separator 3 is significantly damaged due to peel-off of the hydrogen-permeating membrane 3b as well as occurrence of cracks. Damage of the hydrogen-separating pipe 3 means inclusion of C2H2n+2, H2O and CO2 in H2 flowing through the takeout opening 8, resulting in degradation of an objective gas H2.
The present invention aims at provision of a gas reformer which can be driven with higher performance even in case of long-term driving at a high-temperature atmosphere by use of a perforated body made of a ferritic stainless steel containing Cr at a proper ratio in response to a driving temperature.
A new gas reformer proposed by the present invention involves a plurality of hydrogen-separators each having a substrate, which is made of a ferritic stainless steel perforated with holes for passage of H2 gas and coated with a hydrogen-permeating membrane at its external surface. The hydrogen-separators are inserted into inner and outer walls of a double-pipe filled with a catalyst. Hydrocarbon gas is decomposed with combustion heat of a fuel fed into an inner space of the double-pipe, and a decomposition product H2 permeates through the hydrogen-permeating membrane and then flows to the outside.
The ferritic stainless steel, which is used as the substrate for formation of the hydrogen-permeating membrane of the hydrogen-separator to be exposed to an atmosphere of 600-900xc2x0 C., contains 16-25 mass % Cr and Ti and/or Nb at a ratio of (C+N)xc3x978 or more. Ti and/or Nb concentrations are preferably controlled in ranges of 0.1-0.7 mass % Ti and 0.2-0.8 mass % Nb, respectively, under the condition of (Ti, Nb)xe2x89xa7(C+N)xc3x978. The ferritic stainless steel may contain at least one or more of Y and lanthanoids at a ratio up to 0.1 mass % for improvement of oxide resistance, and further contain one or more of Si Mn, AL Mo, Cu, V, W and Ta at a proper ratio for improvement of heat resistance.
The ferritic stainless steel, which is used as the substrate for formation of the hydrogen-permeating membrane of the hydrogen-separator to be exposed to an atmosphere of 450-600xc2x0 C., contains Cr up to 15 mass % and Ti and/or Nb at a ratio of (C+N)xc3x978 or more. Ti and/or Nb concentrations are preferably controlled in ranges of 0.1-0.7 mass % Ti and 0.2-0.8 mass % Nb, respectively, under the condition of (Ti, Nb)xe2x89xa7(C+N)xc3x978.